Gas turbine engines having a core stream surrounded by a bypass duct are well known in the art. Often a fan stage, driven by the turbine through suitable reduction gearing, is disposed upstream of the core stream inlet. The airfoils on these fans were either fixed pitch or variable pitch. Efficient airfoil contours were difficult to design since fan rotation affected the downstream aerodynamics of both the core and bypass stream and sometimes required inconsistent airfoil shapes in front of each stream for most efficient operation. Compromises often had to be made to satisfy the aerodynamics of both streams. Also, variable pitch fans thus located, which are also used to reverse flow through the bypass duct, may adversely affect flow through the core stream during thrust reversing operation since some of the reverse flow through the bypass duct may be blocked by these fan airfoils and turned into the core stream.
Some bypass engines locate the fan airfoil within the bypass stream downstream of the core stream inlet plane. Several U.S. patents representative of such arrangements are: U.S. Pat. Nos. 3,448,582; 3,468,473; 3,673,802; 3,768,933; 3,924,404; and 4,005,575. In the arrangements shown in those patents, the power to rotate the fan rotor is taken off a core engine shaft. The fan disk is supported solely by a torque transmitting drive member located radially inwardly of the core stream inner flow path wall. All the centrifugal loads created by the rotating airfoils as well as the aircraft maneuver loads on the airfoils and the airfoil disks pass across the core stream flow path and are taken out through core stream wall and structural members. Large loads taken out in this manner can create undesireable deflection in seals and other rotating members within the core stream. These deflections often require the engine to be designed with greater gaps at seals and with larger blade tip clearances, which reduce engine efficiency. These problems increase in magnitude with greater bypass ratios (i.e., the ratio of the bypass mass flow rate to the core stream mass flow rate) requiring larger and heavier fan blades which are more difficult to support.